Bottom Line:
How this is achieved remains unclear.Our experiments reveal that both PIN1 localization and microtubule array orientation are likely to respond to a shared upstream regulator that appears to be biomechanical in nature.Lastly, through mathematical modeling we show that such a biophysical coupling could mediate the feedback loop between auxin and its transport that underlies plant phyllotaxis.

Affiliation: Division of Biology, California Institute of Technology, Pasadena, California, United States of America.

ABSTRACTMorphogenesis during multicellular development is regulated by intercellular signaling molecules as well as by the mechanical properties of individual cells. In particular, normal patterns of organogenesis in plants require coordination between growth direction and growth magnitude. How this is achieved remains unclear. Here we show that in Arabidopsis thaliana, auxin patterning and cellular growth are linked through a correlated pattern of auxin efflux carrier localization and cortical microtubule orientation. Our experiments reveal that both PIN1 localization and microtubule array orientation are likely to respond to a shared upstream regulator that appears to be biomechanical in nature. Lastly, through mathematical modeling we show that such a biophysical coupling could mediate the feedback loop between auxin and its transport that underlies plant phyllotaxis.

pbio-1000516-g003: PIN1 behavior in the absence of microtubules.(A) PIN1 behavior at the meristem surface after oryzalin treatment. The absence of cell division and the enlargement of the cells are consistent with the impact of microtubule depolymerization on growth. Nevertheless, differential expression of PIN1 is maintained, and new peaks of PIN1 expression arise 20 h (middle panel) and 47 h (right panel) after microtubule depolymerization. The red dot marks the same cell at the three time points. Scale bar: 50 µm. (B) Close-up of the surface of a meristem expressing PIN1-GFP 67 h after microtubule depolymerization. The PIN1 signal is weaker in the boundary and is polarized in a divergent pattern, as observed in untreated meristems. Scale bar: 20 µm. (C) Transverse sections through a PIN1-GFP meristem treated with oryzalin. As primordia arise, the GFP signal is also detected at sites where the provasculature is initiated (arrowhead), as observed in unteated meristems. Scale bar: 50 µm. (D) Kinetics of PIN1-GFP reorientation at the surface of an oryzalin-treated meristem. The GFP signal (grayscale) is color coded (upper panels) and represented as histograms (lower panels) to better visualize the differences in GFP signals from one time point to another (from left to right 21 h, 27 h, and 42 h after oryzalin treatment). The GFP signal switches from one side of the cell to another, showing that PIN1 retains the ability to reorient in the absence of microtubules. (E) Close-ups of the surface of the same meristem expressing PIN1-GFP before and 47 h after oryzalin treatment. The PIN1-GFP signal becomes broader within the cell after long-term oryzalin treatment. The red dot marks the same cell at the two time points.

Mentions:
The correlations between PIN1 polarities and microtubule array orientations suggest the possibility of a direct causal connection. Although previous studies have shown that microtubules are not directly required for polar PIN1 targeting [13],[14], it has been demonstrated that the microtubule array plays an indirect role in orienting PIN1 polarity [14]. To further investigate this relationship in the SAM we examined PIN1 behavior in the SAM epidermis after depolymerization of the microtubules using oryzalin. As described previously, meristems continue to grow after oryzalin treatment, with the size of the cells also increasing due to the absence of cytokinesis [15],[16]. Oryzalin treatment of PIN1-GFP-expressing meristems revealed that differential PIN1-GFP expression and localization are maintained in the absence of microtubules. Notably, local regions of intense PIN1 signal could be identified in the epidermis and internal layers of the SAM (Figure 3A–3C). These regions corresponded to the position of new primordia (Figure3A and 3B) and concomitant provascular development (Figure 3C). The PIN1-GFP signal was not homogenously distributed on the membrane, notably in the boundary domain where PIN1 polarities localize both towards and away from the developing primordium, as observed in control meristems (Figure 3B; [5]). Lastly, when conducting time-lapse imaging of the PIN1-GFP signal, we observed that PIN1 recruitment could shift from one membrane to another several days after oryzalin treatment (Figure 3D). Altogether, these data are consistent with previous reports showing that phyllotaxis is not altered several days after oryzalin treatment [11],[15], and demonstrate that the general patterns of PIN1 localization found in the SAM are not directly dependent on microtubule orientation.

pbio-1000516-g003: PIN1 behavior in the absence of microtubules.(A) PIN1 behavior at the meristem surface after oryzalin treatment. The absence of cell division and the enlargement of the cells are consistent with the impact of microtubule depolymerization on growth. Nevertheless, differential expression of PIN1 is maintained, and new peaks of PIN1 expression arise 20 h (middle panel) and 47 h (right panel) after microtubule depolymerization. The red dot marks the same cell at the three time points. Scale bar: 50 µm. (B) Close-up of the surface of a meristem expressing PIN1-GFP 67 h after microtubule depolymerization. The PIN1 signal is weaker in the boundary and is polarized in a divergent pattern, as observed in untreated meristems. Scale bar: 20 µm. (C) Transverse sections through a PIN1-GFP meristem treated with oryzalin. As primordia arise, the GFP signal is also detected at sites where the provasculature is initiated (arrowhead), as observed in unteated meristems. Scale bar: 50 µm. (D) Kinetics of PIN1-GFP reorientation at the surface of an oryzalin-treated meristem. The GFP signal (grayscale) is color coded (upper panels) and represented as histograms (lower panels) to better visualize the differences in GFP signals from one time point to another (from left to right 21 h, 27 h, and 42 h after oryzalin treatment). The GFP signal switches from one side of the cell to another, showing that PIN1 retains the ability to reorient in the absence of microtubules. (E) Close-ups of the surface of the same meristem expressing PIN1-GFP before and 47 h after oryzalin treatment. The PIN1-GFP signal becomes broader within the cell after long-term oryzalin treatment. The red dot marks the same cell at the two time points.

Mentions:
The correlations between PIN1 polarities and microtubule array orientations suggest the possibility of a direct causal connection. Although previous studies have shown that microtubules are not directly required for polar PIN1 targeting [13],[14], it has been demonstrated that the microtubule array plays an indirect role in orienting PIN1 polarity [14]. To further investigate this relationship in the SAM we examined PIN1 behavior in the SAM epidermis after depolymerization of the microtubules using oryzalin. As described previously, meristems continue to grow after oryzalin treatment, with the size of the cells also increasing due to the absence of cytokinesis [15],[16]. Oryzalin treatment of PIN1-GFP-expressing meristems revealed that differential PIN1-GFP expression and localization are maintained in the absence of microtubules. Notably, local regions of intense PIN1 signal could be identified in the epidermis and internal layers of the SAM (Figure 3A–3C). These regions corresponded to the position of new primordia (Figure3A and 3B) and concomitant provascular development (Figure 3C). The PIN1-GFP signal was not homogenously distributed on the membrane, notably in the boundary domain where PIN1 polarities localize both towards and away from the developing primordium, as observed in control meristems (Figure 3B; [5]). Lastly, when conducting time-lapse imaging of the PIN1-GFP signal, we observed that PIN1 recruitment could shift from one membrane to another several days after oryzalin treatment (Figure 3D). Altogether, these data are consistent with previous reports showing that phyllotaxis is not altered several days after oryzalin treatment [11],[15], and demonstrate that the general patterns of PIN1 localization found in the SAM are not directly dependent on microtubule orientation.

Bottom Line:
How this is achieved remains unclear.Our experiments reveal that both PIN1 localization and microtubule array orientation are likely to respond to a shared upstream regulator that appears to be biomechanical in nature.Lastly, through mathematical modeling we show that such a biophysical coupling could mediate the feedback loop between auxin and its transport that underlies plant phyllotaxis.

Affiliation:
Division of Biology, California Institute of Technology, Pasadena, California, United States of America.

ABSTRACTMorphogenesis during multicellular development is regulated by intercellular signaling molecules as well as by the mechanical properties of individual cells. In particular, normal patterns of organogenesis in plants require coordination between growth direction and growth magnitude. How this is achieved remains unclear. Here we show that in Arabidopsis thaliana, auxin patterning and cellular growth are linked through a correlated pattern of auxin efflux carrier localization and cortical microtubule orientation. Our experiments reveal that both PIN1 localization and microtubule array orientation are likely to respond to a shared upstream regulator that appears to be biomechanical in nature. Lastly, through mathematical modeling we show that such a biophysical coupling could mediate the feedback loop between auxin and its transport that underlies plant phyllotaxis.